Vapor generator



Jan. 26, 1965 T. R. BEST, JR

VAPOR GENERATOR 3 Sheets-Sheet l Filed Nov. l5, 1962 mm x /NVfA/Ta/e THEODORE E. 5557,' .77

Jan. 26, 1965 T. R, BEST, JR 3,166,909

VAPOR GENERATOR Filed Nov. 15, 1962 5 Sheets-Sheet 2 58a. R CE 62 IIIII I lo ,f N 2 2 3 3 .I. I

29 SUPER- 72 74 28 Rural-HEME'` 5s i 32,

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94 CONTROL A Tra/@Mfr T. R. BEST, JR

VAPOR GENERATOR Jan. 26, 1965 5 Sheets-Sheet I5 Filed NOV. 15, 1962 esunocoonsansllnl lirlIFllnL United States Patent Office 3,166,909 VAPOR GENERATOR Theodore R. Best, Jr., Westwood, NJ., assigner to Foster Wheeler Corporation, New York, N.Y., a corporation i of New York v Filed Nov. 15, 1962, Ser. No. 237,982

7 Claims. (Cl. 60-73) This invention relates to a force ow, once-through vapor generator, and more particularly, to a supercritical vap'or generator having turbine stages in series at a pluralit-y of pressure levels with separate stages of reheat for each intermediate pressure turbine.

Supercritical steam generating units today generally require in addition to ahigh pressure and a low pressure turbine, at' least one intermediate pressure turbine. In order to insure the supply of dry steam to the intermediate and'low pressure turbines, it is necessary to have two stages of reheat of turbine exhaust steam. Since the requirements for the reheat intermediate and low pressure turbines will differ from each other, it is necessary to provide separate reheat circuits in the generator for the stages of reheat.

However, several problems are then presented. For instance, it is essential to maintain constant inlet temperatures to Vthe intermediate or reheat turbines during load changes or uctuations, which fluctuations may be as much as 50% of full load. This is difcult since the temperature of the vapor liuid will vary with changes in ow rate, and -requires that means be provided for separately controlling the heat input into each vapor reheat stage. Control of the heat input into each vapor reheat stage also is-important in that the first stage reheat duty will be a greater percentage of the tot-al output as compared to the second stage reheat. As an example, the first or high pressure reheat may provide 13.5% of the total output, and the second or low pressure reheat, 9.65% of the total output.

'Also, during start-up of the generator, it is necessary to isolate the reheaters and later to control the reheat temperatures. For instance, a start-up sequence may require yinitially putting most of the gas iiow through the superheater,v with subsequent changeover of gas flow through one of the 'reheaters, and then through theother, after generation of steam by the superheater. This requires isolation of the reheaters, not only from each other, but also from the superheater.

These and otherdiiculties are met in accordance with the invention by -a novel vapor generator arrangement or unit which comprises, in combination with a plurality of turbines including 4a high pressure turbine and at least first andv second intermediate pressure turbines, means defining first and second upright furnace sections and means formingiirst and second gas passes leading from each of the furnace sections. Superheater tubes are disposed 'in each of the said rst gas passes anda first and a second reheater means, constituting first (primary) and second reheat circuits or stages of reheat for the intermediate pressure turbines, are disposed in each of said second gas passes. Means are provided for baffling or controlling the ow through all of the gas passes, and for differentially firing the furnaces.

In a preferred embodiment, the tubes of the rst or primary reheater circuit or reheat stage aredisposed so as to be subjected to radiant heat transfer as well as heat transfer by convection. This is accomplished by initially, or at a point in the reheat circuit, bringingthe reheater tubes into both vapor generators along the walls of the superheater convection passageway and across the roof of each furnace section. For reasons to be described, this provides with greater facility, constant temperature steam to the iirst intermediate pressure turbine.

i vapor generating wall tubes 16 and 18 respectively. Four o ow of superheated vapor from the superheaters 28 65.; same temperature.

passage, the roof of each furnace section and the roof ofv each reheater convection passage or passageway, and then along a wall Vof the latter passageways. These tubes constitute a first part of the'primary reheat circuit. Thereafter the partially reheated streams are combined to flow into a bank of reheat tubes which are disposed in one of the above-mentioned second gas passes, which tubes form the second half of the primary reheat circuit. After iiow through the primar-y reheater circuit, the

steam is conducted to the rst or primary intermediatey pressure or reheat turbine. The exhaust steam from this turbine is then conducted lto the second convection reheater constituting a second stage or circuit of reheating and located in the second of the above mentioned gas passes, and from there is fed to the second intermediate pressure or reheat turbine.

It will be apparent that by the invention, even at the yhigh pressures contemplated, the second stage reheating will insure that `dry steam is provided to the low pressure turbine. In addition, by differentially tiring the two furnaces plus biasing the gas ow from each furnace selectively through the reheater and superheater sections, optimum flexibility and control of the turbine inlet temperatures is achieved.Y In this latter respect, theturbine 'inlet temperatures can be maintainedV constant despite wide iiuctuations in the turbinerload ranges.

Towards control of the turbine inlet temperatures, an improved result is obtained by,in the first stage of reheating, effecting heat transfer by radiation as well as by convection.

These and other upon consideration of the following specification and accompanying drawings, in which:

FTGURE 1 is a schematic perspective view of a vapor generator assembly in accordance with the invention;

FGURE 2 is a section elevation view of a vapor 4gention; and

FIGURE 3 is a plan section View taken along line 3--3 of FIG. 2'.

` Referring to FIG. 1, the generator comprises adjacent, parallel upright furnace sections 12 and 14 defined by convection gas passes (2t), 22, 24 and 26) lead from the furnaces, two each, the left hand passes 20 and 24 of the two generators housing superheater sections 28 and 30, and the right hand passes 22 and 26 of the geni erators housing first and second reheater tube banks or sections 32 and 34 respectively. Also shown on the drawing are a high pressure turbine 36, intermediate pressure or reheat turbines 38 and 40 and low pressure turbine 42.l

In the two stage reheat circuit of the invention, the

and 30 and superheater outlet headers 29 and 31 is fed by separate lines 44 and 46 past mixing line 48 to stages of the high pressure turbine. The mixing line 48 insures that the streams introduced to the turbine'will'have the The exhaust steam from the high pressure turbine is conducted by lines 50 and 52 to headers 54 and 56 of each vapor generator, the latter being disposed near the outlet ends of each of the superheater convection passageways or gas passes 20 and y24. In the generators, the exhaust steam is then conducted through tubular members 58 and 60 lining a wall, or

Patented Jan. 26, 1965' advantages will become'more apparent Walls, of each of the convection passageways 20 and 24 (housing the superheater sections), which tubes then extending horizontally and transversely along the roofs of the convection passageways for each furnace (tubes designated by the numerals 58a and 60a), and along the roofs of the furnace sections 12 and 14 and the reheater convection passageways 22 and 26 to headers 62 amd64. Following this, the streams are conducted downwardly in tubes 5827 `and V60h along a wall or walls of each of the reheater convection passageways 22 and 26 to outlet headers 66 and 68.

The now partially reheated outputs from the headers are combined inline 69 and fed to a single header 70 serving a bank of tubes 32 in the convection passageway 22, which tubes constitute a second or final part of the iirst stage or primary reheater. The reheated steamvcollected in the reheater outlet header 72 is then introduced through line 74 to a stage of the reheat or intermediate pressure turbine 38.

The exhaust steam from the intermediate pressure turbine is passed by line 76 to header 78 by which the vapor is introduced to the second stage reheater 34, the reheated steam from the outlet header 80 for the reheater being fed by line 82 to the second intermediate pressure or reheat turbine 40. The vapor ow then proceeds by line 84 to the low pressure turbine 42, and from .there to the condensers (not shown) in a conventional manner.

The physical arrangement of the generators should be more apparent from FIGS. 2 and 3, FIG. 2 being an elevation section view Vof the furnace and generating unit 12 (Furnace No. l, FIG. l), FIG. 3 being a plan section view of both ofthe generating units 12 and 14. FIG. 2 shows the furnace 12 having wall :tubes 16, gas passes 20 and 22, superheater 28, superheater outlet header 29, and first stage'reheater convection part 32 with inlet and outlet headers 70 and 72. Also shown as the first part of the first stage reheat circuit are convection tubes 58 extending upwardly along the wall of the superheater convection passageway 20 from header 54, receiving high pressure turbine exhaust steam. These tubes continue horizontally across the roof of the pass (58a), across the roof of the furnace, and across the roof of the reheater gas passageway to header 62. From header 62 the convection tubes extend downwardly alongside the reheater 32to header 66, the header 66 communicating with header 70 through tube 69.

Dampers 90 and 92 are provided at the outlets for the superheater and reheater convection passageways 20 and 22, respectively (-in FIG. 3, the dampers for Furnace No. 2 are designated bythe numerals 9S and 100), and means 94 are provided for controlling the firing rate of each furnace section by burners 96. These permit the steam reheat temperature as well as the high pressure turbine inlet steam temperature to be controlled accurately, i.e., to be maintained constant during start-up and during uctuations in load, rst by differential or controlled firing of the burners, and second by biasing of the gas ow through either `the superheater passageway 20 or :the reheater passageway 22 of each furnace.

At low loads, the gas temperatures entering the reheater convection passageways will be lower because of a proportionately greater heat transfer, compared to full load, in-the furnace sections. It -is well known that when heat is transferred by convection, the rate of transfer is proportional to the gas flow rate and temperature difference, S that it has been past practice to increase bias gas flow over the reheater at lower turbine loads to offset the lower gas entering temperature. However, this is effective only up to the point where an upper limit is reached on the amount of gas that can be directed through lthe reheater section.

It is also well known that when heat is transferred by radiation the temperature of the uid being heated tends to increase with a decrease in turbine load. Accordingly alsace@ combining the effects of radiation and convection in the first reheat stage extends the load range over which a constant temperature fluid can be obtained in the reheater, beyond the range achieved by merely employing convection heat transfer with biasing gas flow.

Since the rst stage reheater duty is a greater percentage of the total output, it generally is necessary to differentially fire the furnaces, i.e., employ an unequal firing rate in each furnace. This ability to differentially fire, however, also serves another purpose in that it provides a means for extending the load range over which a constant temperature can be obtained in the fluid leaving the second stage reheater. It will be recalled that no direct benefit towards extending the load range for the second stage reheater is realized from the radiant sections of the first stage reheatcr. Accordingly, after maximum adjustment has been made in the gas ilow rate, it is a simple mater to overre the second furnace to obtain the desired reheater temperature. To offset the eiect of unequal firing in each furnace upon the superheater temperature, the feedwater iiow may be increased or decreased to each furnace so that the control of feedwater flow will in turn be proportional to the firing rate in each furnace.

The characteristics of radiant heat transfer are obtained by disposing at least a portion of the tube section 58 (also tube section 6), not shown in FIG. 2) of the first stage reheater in a radiant portion of the furnace (tubes 56a and 60a), and balancing the tube surfaces so that the convection and radiant heat transfer curves tend to cancel out. The particular tube surface areas required to obtain a more constant temperature at the first reheat turbine inlet throughout the operating range of the geuerator is a matter of design.

Referring to FIG. 3, furnaces 12 and 14 are shown with the appropriate reheater circuit How connections. In particular, on the superheater sides of the generator, ows from the superheater headers 29 and 31 are combined or mixed by joining lines 44 and 46 for flow to the high pressure turbine. Exhaust fiow from the high pressure turbine is returned to both generators in lines 50 and 52 :to headers 54 and 56 serving a first part of the primary reheat stage (tubes 5S and 60).

Adjacent to the reheater convection sections (the primary reheater bank 32 and secondary reheater 34) the partially reheated streams from headers 66 and 68 from each generator are combined in line 69 and fed to header 70 serving the remainder of the primary reheater, tube bank 32 of the primary generator 12. The reheated steam from reheater outlet header 72 is then fed to the primary reheat turbine (line 74).

The exhaust steam from the rst reheat turbine is shown as passing, by line 76, to header '78 serving the bank of reheat tubes 34 disposed in the gas pass 26 of the second generator. Outlet headers and line 82 lead to the second reheat turbine.

It should now be apparent that a principal advantage of the invention resides in the provision of first and second reheat sections under conditions Iby which optimum efficiency and control is obtained. In particular, the provision of separate gas passes for the superheater separate from those for the reheaters permits baiiiing and/or biasing the passes to provide to the turbines vapor having the exact temperatures required, even under conditions of wide fluctuations in load. Further control is achieved by having separate furnaces for the reheaters which may be differentially fired. Diterential firing also achieves the control with maximum eiiiciency or utilization of heat. An incidental advantage is that the second reheat section insures that dry steam will be presented to the low pressure turbine.

Further, during start-up or shut-down, the reheaters may be completely or partially isolated as desired.

Other advantages of the invention and modifications thereof will be apparent to those skilled in the art and within the scope and spirit of the invention.

aisance What is claimed is:

1. A vapor generating and superheating unit of the once-through supercritical type comprising a plurality of vapor generating tubes defining first and second upright furnace sections, Wall means forming first and second gas passes leading from said first furnace section and first and second gas passes leading from said second furnace section, superheater means in said first gas passes of both said first and second sections and reheater means in said second gas passes of said first and second sections, means to control the gas fiow from beth said furnace sections through said gas passes, and means to control the firing to both said furnace sections.

2. In combination with a plurality of turbines including a high pressure turbine and at least first and second intermediate pressure turbines, a vapor generating and superheating unit of the once-through supercritical type comprising a plurality of vapor generating tubes defining first and second furnace sections, Wall means defining first and second gas passes leading from each of said furnace sections, superheater tubes disposed in each of said first gas passes and first and second reheater means disposed separately in said second gas passes, means conveying the flow from the superheater tubes to the high pressure turbine and from the turbine to the first reheater means, to the first intermediate pressure turbine, to the second reheater means and to the second intermediate pressure turbine in that order, means to control the gas fiow through each of said gas passes, and means to control the firing to each of said furnace sections.

3. A vapor generating and superheating unit according to claim 2 and further comprising means to convey the fluid from the high pressure turbine along Wall portions of the vapor generating unit exposed to radiant energy whereby the fluid supplied to the first intermediate pressure turbine is heated by radiation as Well as by convection.

4. In combination with a high pressure turbine and at least first and second intermediate pressure turbines, a Vapor generator circuit for a once-through supercritical unit which comprises means defining at least two furnace sections for the generator, superheater tubes and first and second reheater tubes, means to convey the fluid fiow from said superheater tubes to said high pressure turbine, and from said turbine to said first reheater tubes, to said first intermediate pressure turbine, to said second reheater tubes and to said second intermediate pressure turbine in that order, said superheater tubes and first and second reheater tubes being located in separate gas passes leading from the generator furnace, means to control the gas fiow through said separate gas passes,

and means to control the firing to each of said furnace sections.

5. A method for reheating a plurality of vapor streams in series, following expansion and cooling of each of the vapor streams, comprising passing said vapor streams through separate heating zones which receive gas streams emanating from separately fired furnace sections, said heating zones being in parallel with other heating zones also receiving gas emanating from said furnace sections, separately controlling the firing rates to said furnace sections, and biasing the gas tiows through one or the other of said parallel heating zones.

6. A method according to claim 5, wherein the heat transfer in said heating zones is by convection, further comprising the step of subjecting at least one of said vapor streams to radiant heat transfer from said furnace sections, the characteristics of radiant heat transfer extending the load range for Said Vapor stream over which a constant temperature vapor is obtained beyond that achieved by merely employing convection heat transfer with biasing gas flow.

7. in combination with a plurality of turbines including a high pressure t-urbine and at least first and second intermediate pressure turbines, a vapor generating and superheating unit of the once-through supercritical type comprising a plurality of vapor generating tubes defining first and second furnace sections, wall means defining first and second gas passes leading from each of said furnace sections, superheater tubes disposed in each of said first gas passes and first and second reheater means disposed in each of said second gas passes, means conveying the flow from the superheater tubes to the high pressure turbine and from the turbine to the first reheater means, to the first intermediate pressure turbine, to the second reheater means and to the second intermediate pressure turbine in that order, means to control the gas flow through each of said gas passes and means to control the firing to each of said furnaces, said first reheater means further comprising tubes adjacent the Wall means defining said first and second gas passes, portions of said tubes passing along the roofs of said furnace sections.

References Cited by the Examiner UNTTED STATES PATENTS 8/53 Van Brunt 122--479 9/59 Buri 60-73 

1. A VAPOR GENERATING AND SUPERHEATING UNIT OF THE ONCE-THROUGH SUPERCRITICAL TYPE COMPRISING A PLURALITY OF VAPOR GENERATING TUBES DEFINING FIRST AND SECOND UPRIGHT FURNACE SECTIONS, WALL MEANS FORMING FIRST AND SECOND GAS PASSES LEADING FROM SAID FIRST FURNACE SECTION AND FIRST AND SECOND GAS PASSES LEADING FROM SAID SECOND FURNACE SECTION, SUPERHEATER MEANS IN SAID FIRST GAS PASSES OF BOTH SAID FIRST AND SECOND SECTION AND REHEATER MEANS IN SAID SECOND GAS PASSES OF SAID FIRST AND SECOND SECTIONS, MEANS TO CONTROL THE GAS FLOW FROM BOTH SAID FURNACE SECTIONS THROUGH SAID GAS PASSES, AND MEANS TO CONTROL THE FIRING TO BOTH SAID FURNACE SECTIONS. 